Backside sensing biofet with enhanced performance

ABSTRACT

The present disclosure provides a bio-field effect transistor (BioFET) and a method of fabricating a BioFET device. The method includes forming a BioFET using one or more process steps compatible with or typical to a complementary metal-oxide-semiconductor (CMOS) process. The BioFET device includes a substrate, a transistor structure having a treated layer adjacent to the channel region, an isolation layer, and a dielectric layer in an opening of the isolation layer on the treated layer. The dielectric layer and the treated layer are disposed on opposite side of the transistor from a gate structure. The treated layer may be a lightly doped channel layer or a depleted layer.

RELATED CASES

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/785,055, filed Mar. 14, 2013, and entitled “Backside SensingBioFET with Enhanced Performance,” which application is incorporatedherein by reference.

FIELD

This disclosure relates to biosensors and methods for forming bio-chips.Particularly, this disclosure relates to bio-chips having biosensors andfluidic devices and methods for forming them.

BACKGROUND

Biosensors are devices for sensing and detecting biomolecules andoperate on the basis of electronic, electrochemical, optical, andmechanical detection principles. Biosensors that include transistors aresensors that electrically sense charges, photons, and mechanicalproperties of bio-entities or biomolecules. The detection can beperformed by detecting the bio-entities or biomolecules themselves, orthrough interaction and reaction between specified reactants andbio-entities/biomolecules. Such biosensors can be manufactured usingsemiconductor processes, can quickly convert electric signals, and canbe easily applied to integrated circuits (ICs) andmicroelectromechanical systems (MEMS).

Biochips are essentially miniaturized laboratories that can performhundreds or thousands of simultaneous biochemical reactions. Biochipscan detect particular biomolecules, measure their properties, processthe signal, and may even analyze the data directly. Biochips enableresearchers to quickly screen large numbers of biological analytes insmall quantities for a variety of purposes, from disease diagnosis todetection of bioterrorism agents. Advanced biochips use a number ofbiosensors along with microfluidics to integrate reaction, sensing andsample management. BioFETs (biological field-effect transistors, orbio-organic field-effect transistors) are a type of biosensor thatincludes a transistor for electrically sensing biomolecules orbio-entities. While BioFETs are advantageous in many respects,challenges in their fabrication and/or operation arise, for example, dueto compatibility issues between the semiconductor fabrication processes,the biological applications, restrictions and/or limits on thesemiconductor fabrication processes, sensitivity and resolution of theelectrical signals and biological applications, and/or other challengesarising from implementing a large scale integration (LSI) process.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 is a cross-sectional view of an embodiment of a BioFET deviceaccording to one or more aspects of the present disclosure.

FIGS. 2A and 2B are flow charts of various embodiments of a method offabricating a BioFET device according to one or more aspects of thepresent disclosure.

FIGS. 3-14 are cross-sectional views of various embodiments of a BioFETdevice constructed according to the present disclosure.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof the invention. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Moreover,the formation of a first feature over or on a second feature in thedescription that follows may include embodiments in which the first andsecond features are formed in direct contact, and may also includeembodiments in which additional features may be formed interposing thefirst and second features, such that the first and second features maynot be in direct contact. Further still, references to relative termssuch as “top”, “front”, “bottom”, and “back” are used to provide arelative relationship between elements and are not intended to imply anyabsolute direction. Various features may be arbitrarily drawn indifferent scales for simplicity and clarity.

In a BioFET, the gate of a MOSFET (metal-oxide-semiconductorfield-effect transistor), which controls the conductance of thesemiconductor between its source and drain contacts, is replaced by abio- or biochemical-compatible layer or a biofunctionalized layer ofimmobilized probe molecules that act as surface receptors. Essentially,a BioFET is a field-effect biosensor with a semiconductor transducer. Anadvantage of BioFETs is the prospect of label-free operation. Use ofBioFETs avoids costly and time-consuming labeling operations suchtagging analytes with fluorescent or radioactive probes.

Binding of a target biomolecule or bio-entity to the gate or a receptormolecule immobilized on the gate of the BioFET modulates the conductanceof the BioFET. When the target biomolecule or bio-entity is bonded tothe gate or the immobilized receptor, the drain current of the BioFET isvaried by the gate potential, which depends on the type and amount oftarget bound. This change in the drain current can be measured and usedto determine the type and amount of the bonding between the receptor andthe target biomolecule or the biomolecule itself. A variety of receptorsmay be used to functionalize the gate of the BioFET such as ions,enzymes, antibodies, ligands, receptors, peptides, oligonucleotides,cells of organs, organisms and pieces of tissue. For instance, to detectssDNA (single-stranded deoxyribonucleic acid), the gate of the BioFETmay be functionalized with immobilized complementary ssDNA strands.Also, to detect various proteins such as tumor markers, the gate of theBioFET may be functionalized with monoclonal antibodies.

One example of a biosensor has a sensing surface as a top of a floatinggate connected to the gate of the BioFET. The floating gate is connectedto the gate structure of the BioFET through a stack of metalinterconnect lines and vias (or multi-layer interconnect, MLI). Thevarious metal layers over the gate electrode can also contribute todamage by antenna effect during the MLI formation process. In such aBioFET, the potential-modulating reaction takes place at an outersurface of the final (top) metal layer or a dielectric surface formed ontop of the MLI and is sensed indirectly by the BioFET. The sensitivityof the device is lower than other biosensors because of parasiticcapacitances associated with the MLI. As result a sensing platedimension is usually specified so that a sufficiently detectable amountof potential-modulating reaction can take place on the sensing plate.The minimum sensing plate dimension in turn limits the BioFET density.

In another example, the biomolecules bind directly or through receptorson the gate or the gate dielectric of the BioFET. These “direct sensing”BioFETs directly senses the target biomolecules without the parasiticcapacitances associated with MLI. Its construction requires removal ofthe MLI material above the BioFET to form a sensing well and exposes thegate electrode or gate dielectric to the fluidic environment wherepotential-modulating surface reactions occur. These BioFETs are moresensitive than the floating gate types but are challenging to constructfor several reasons. The sensing well etched has a high aspect ratio,for example, 30 or greater, so it is usually performed with high energyplasma etch. The high-aspect ratio of the sensing well also limits theprofile of the etched sensing well. The high energy plasma etch candamage the gate electrode due to charge-induced damage. One attempt inreducing the aspect ratio of the sensing well to make the etch easierresults in limitation of the number of metal layers, down to one or twometal layers. The reduction in metal layers limits the interconnectrouting and integration options of the device, for example, the numberand type of circuits for controlling the BioFET. The process is alsovery sensitive to alignment, because misalignment may expose the metalsin the MLI surrounding sensing well or cause the sensing surface area tobe smaller than designed.

In yet another example, the biomolecules are placed close to the gate ona backside of the substrate. In this example, a gate and sensing surfaceare formed on the backside of the channel region through backside of thesubstrate as a fluidic gate. This example avoids the difficulty ofhaving to etch through multiple layers of interconnects and yet placingthe biomolecules proximate to the gate to have much higher sensitivitythan the floating gate biosensor. This type of BioFET is referred to asthe backside sensing (BSS) BioFET. The various embodiments of thepresent disclosure involve a BSS BioFET that includes a dopantconcentration gradient in the active region under the gate between thesource and drain and/or a surface treatment of the active region surfaceproximate to the fluidic gate. Such dopant concentration gradient allowselectrical property tuning of the BSS BioFET. The active region includesa treated layer proximate to the fluidic gate and a channel region. Thedopant concentration gradient may be a lightly doped layer or a depletedlayer formed by adding a dopant of a different conductivity type fromthe rest of the channel region to a treated layer of the channel regionor by deactivating dopants in a thin treated layer of the channelregion. Surface treatments also include annealing under oxygen orhydrogen environments.

FIG. 1 is a schematic drawing of a backside sensing (BSS) BioFET 100.The semiconductor device 100 includes a gate structure 102 formed onsubstrate 114. The gate structure 102 is a back gate for the BSS BioFET.The substrate 114 further includes a source region 104, a drain region106, and an active region 108 (e.g., including a channel region)interposing the source region 104 and the drain region 106. The gatestructure 102, the source region 104, the drain region 106, and theactive region 108 may be formed using suitable CMOS process technology.The gate structure 102, the source region 104, the drain region 106, andthe active region 108 form a FET. A portion of the active region 108proximate to the backside is a treated layer 107, which may be a lightlydoped channel layer or a depleted layer. The treated layer 107 mayinclude dopants not found in the rest of the active region 108. Forexample, for a n-MOS, the treated layer 107 may be doped with arsenic orphosphorous. For a p-MOS, the treated layer 107 may be doped with boron.The treated layer 107 may include neutralizing species that tends todeactivate dopants, for example, hydrogen to deactivate boron. Thetreated layer 107 may be formed by annealing to repair dangling bonds orto mitigate plasma induced defects. Anneals in an oxygen atmosphere ofoxygen or ozone repairs dangling bonds. Anneals in a hydrogen atmosphereof hydrogen or deuterium mitigates mobile ions and interfacial trapsfrom plasma-induced damage.

An isolation layer 110 is disposed on the opposing side of the substrate114, as compared to the gate structure 102. The isolation layer 110 maybe a buried oxide (BOX) layer of a silicon-on-insulator (SOI) substrate.An opening in the isolation layer 110 is substantially aligned with theactive region 108. A dielectric layer 124 is disposed on the bottom ofthe opening on the back surface of the active region 108. The dielectriclayer 124 functions as the gate dielectric for the fluidic gate andcovers the surface of the treated layer 107 as well as any portion ofthe source and drain (106/104) not covered by the isolation layer 110.

In some embodiments, a metal crown structure 126 is disposed over thedielectric layer 124 and at least partially covering the sidewalls ofthe isolation layer 110. When used, the metal crown structure 126 is thesensing surface used to detect biomolecules or bio-entities. Area of themetal crown structure 126 is larger than the dielectric layer 124 andthus can accommodate more potential modulating reactions. In someembodiments, the metal crown structure 126 extends over the top cornersof the opening in the isolation layer 110 and partially covers theisolation layer 110. In certain embodiments, a number of receptors arebound or amplified on the metal crown structure 126 to provide sites fordetecting biomolecules or bio-entities. In other embodiments, the metalcrown structure 126 surface is used to bind biomolecules or bio-entities128 having particular affinities to the metal material. Themetal-containing material for metal crown structure 126 includestantalum, tantalum nitride, niobium, tungsten nitride, ruthenium oxide,or combinations of these. Other metals including gold and platinum mayalso be used. According to some embodiments, the material for the metalcrown structure 126 is an ohmic metal. The semiconductor device 100includes electrical contacts (not shown) to the source region 106, thedrain region, the gate structure 102, and the gate via the metal crownstructure 126. If the metal crown structure 126 is not used, then thedielectric layer 124 is an interface layer that provides binding sitesfor receptors.

Thus, while a conventional FET uses a gate contact to controlconductance of the semiconductor between the source and drain (e.g., thechannel), the semiconductor device 100 allows receptors formed on thebackside of the FET device to control the conductance, while the gatestructure 102 (e.g., polysilicon) acts as a back gate (e.g., sourcesubstrate or body node in a conventional FET). The back gate can controlthe channel electron distribution without a bulk substrate effect. Thus,if molecules attach to receptors on the fluidic gate, the resistance ofthe field-effect transistor channel region is altered. Either gate maybe biased. A front fluidic gate electrode is located proximate to thesensing surface on the metal crown structure or on the interface layer.Therefore, the semiconductor device 100 may be used to detect one ormore specific biomolecules or bio-entities in the analyte environment130 contained in the fluidic structure 132.

By adding dopants to the treated layer 107 under the dielectric layer124, the performance of the BioFET 100 may be tuned. According tovarious embodiments, when the treated layer 107 is a lightly doped layeror a depleted layer, the BioFET 100 can be made more sensitive tomolecules bound to receptors or to the gate. In other words, the draincurrent for a gate voltage may be increased relative to a BioFET withouttreated layer 107. In some embodiments, the treated layer 107 provides alarger bandgap which can avoid or reduce current leakage.

The semiconductor device 100 may include additional passive componentssuch as resistors, capacitors, inductors, and/or fuses; and other activecomponents, including P-channel field effect transistors (pFETs),N-channel field effect transistors (nFETs), metal-oxide-semiconductorfield effect transistors (MOSFETs), complementarymetal-oxide-semiconductor (CMOS) transistors, high voltage transistors,and/or high frequency transistors. It is further understood thatadditional features can be added in the semiconductor device 100, andsome of the features described below can be replaced or eliminated, foradditional embodiments of the semiconductor device 100.

FIG. 2A is process flow diagram of a method 200 for making a BSSbiological field effect transistor (BioFET). The method 200 includesforming a BioFET using one or more process operations compatible with ortypical of complementary metal-oxide-semiconductor (CMOS) process. It isunderstood that additional steps can be provided before, during, andafter the method 200, and some of the steps described below can bereplaced or eliminated in different embodiments of the presentdisclosure. Further, it is understood that the method 200 includes stepshaving features of a typical CMOS technology process flow and those areonly described briefly herein.

The method 200 begins at operation 202 where a substrate is provided.The substrate is a semiconductor substrate. The semiconductor substratemay be a silicon substrate. Alternatively, the substrate may compriseanother elementary semiconductor, such as germanium; a compoundsemiconductor including silicon carbide, gallium arsenic, galliumphosphide, indium phosphide, indium arsenide, and/or indium antimonide;an alloy semiconductor including SiGe, GaAsP, AlinAs, AlGaAs, GaInAs,GaInP, and/or GaInAsP; or combinations thereof. In various embodiments,the substrate is a semiconductor-on-insulator (SOI) substrate. The SOIsubstrate may include a buried oxide (BOX) layer formed by a processsuch as separation by implanted oxygen (SIMOX), and/or other suitableprocesses. The substrate may be doped, such as p-type and n-type. Asused herein, workpiece refers to a substrate together with any materialbonded or deposited thereon. The semiconductor substrate (or devicesubstrate) refers to the base material on and in which the devices arebuilt and does not include any deposited or bonded material. FIG. 3 is across section of a partially fabricated BioFET 300 having a substrate302. In the example of FIG. 3, the substrate 302 is an SOI substrateincluding a bulk silicon layer 304, an oxide layer 306, and an activelayer 308. The oxide layer 306 may be a buried oxide (BOX) layer. In anembodiment, the BOX layer is silicon dioxide (SiO2). The active layer308 may include silicon. The active layer 308 may be suitably doped withn-type and/or p-type dopants.

Referring to FIG. 2A, the method 200 then proceeds to operation 204where a field effect transistor (FET) is formed on the substrate. TheFET may be an n-type FET (nFET) or a p-type FET (pFET). The FET includesa gate structure, a source region, a drain region, and a channel regionbetween the source and drain regions. For example, the source/drainregions may comprise n-type dopants or p-type dopants depending on thetype of FET. The gate structure includes a gate dielectric layer, a gateelectrode layer, and/or other suitable layers. In some embodiments, thegate electrode is polysilicon. Other gate electrodes include metal gateelectrodes including material such as, Cu, W, Ti, Ta, Cr, Pt, Ag, Au,suitable metallic compounds like TiN, TaN, NiSi, CoSi, or combinationsof these conductive materials. In various embodiments, the gatedielectric is silicon oxide. Other gate dielectrics include siliconnitride, silicon oxynitride, a dielectric with a high dielectricconstant (high k), and/or combinations thereof. Examples of high kmaterials include hafnium silicate, hafnium oxide, zirconium oxide,aluminum oxide, tantalum pentoxide, hafnium dioxide-alumina (HfO2-Al2O3)alloy, or combinations thereof. The FET may be formed using typical CMOSprocesses such as, photolithography; ion implantation; diffusion;deposition including physical vapor deposition (PVD), metal evaporationor sputtering, chemical vapor deposition (CVD), plasma-enhanced chemicalvapor deposition (PECVD), atmospheric pressure chemical vapor deposition(APCVD), low-pressure CVD (LPCVD), high density plasma CVD (HDPCVD),atomic layer CVD (ALCVD), spin on coating; etching including wetetching, dry etching, and plasma etching; and/or other suitable CMOSprocesses.

FIG. 3 is a cross section of a partially fabricated BioFET 300 having asubstrate 302. The partially fabricated BioFET 300 includes a gatedielectric 312, a gate electrode 314, source/drain regions 316, andactive region 319. The source/drain regions 316 and the active region319 may include opposite-type (e.g., n-type/p-type) dopants. The gateelectrode 314 is a polysilicon gate or a metal gate. The gate dielectric312 is a gate oxide layer (e.g., SiO2, HfO2, or other high k metaloxide).

After forming the FET on the substrate, a multi-layer interconnect (MLI)structure is formed on the substrate. The MLI structure may includeconductive lines, conductive vias, and/or interposing dielectric layers(e.g., interlayer dielectric (ILD)). The MLI structure may providephysical and electrical connection to the transistor. The conductivelines may comprise copper, aluminum, tungsten, tantalum, titanium,nickel, cobalt, metal silicide, metal nitride, poly silicon,combinations thereof, and/or other materials possibly including one ormore layers or linings. The interposing or inter-layer dielectric layers(e.g., ILD layer(s)) may comprise silicon dioxide, fluorinated siliconglass (FGS), SILK (a product of Dow Chemical of Michigan), BLACK DIAMOND(available from Applied Materials of Santa Clara, Calif.), and/or otherinsulating materials. The MLI may be formed by suitable processestypical in CMOS fabrication such as CVD, PVD, ALD, plating, spin-oncoating, and/or other processes.

Referring to the example of FIG. 3, an MLI structure 318 is disposed onthe substrate 302. The MLI structure 318 includes a plurality ofconductive lines 320 connected by conductive vias or plugs 322. In anembodiment, the conductive lines 320 include aluminum and/or copper. Inan embodiment, the vias 322 include tungsten. In another embodiment, thevias 322 include copper. A dielectric layer 324 is disposed on thesubstrate 302 including interposing the conductive features of the MLIstructure 318. The dielectric layer 324 may be an inter-layer dielectric(ILD layer) or an inter-metal dielectric (IMD) layer and/or composed ofmultiple ILD or IMD sub-layers. In an embodiment, the dielectric layer324 includes silicon oxide. The MLI structure 318 provides electricalconnection to the gate 314 and/or the source/drain 316.

Referring back to FIG. 2A, in operation 206, an opening is formed at thebackside of the substrate. The opening is a trench formed in one or morelayers disposed on the backside of the substrate. The opening exposes aregion of the substrate underlying the gate and adjacent to the channelregion of the FET. The opening may be formed using suitablephotolithography processes to provide a pattern on the substrate andetching processes to remove materials form the backside until the bodystructure of the FET device is exposed. Suitable etching processesinclude wet etch, dry etch, including plasma etch and/or other suitableprocesses.

In some embodiments, details of the forming the opening operationincludes a number of steps as shown in process diagram of FIG. 2B andcross sections of FIGS. 4 to 10. In operation 252 of FIG. 2B, a carriersubstrate is attached. As shown in FIG. 4, a carrier substrate 402 isattached (e.g., bonded) to the device substrate 302. The carriersubstrate 402 is attached to the front side of the device substrate 302over the MLIs. In an embodiment, the carrier substrate is bonded to apassivation layer 404 formed on the MLI and/or ILD layers of thesubstrate. The carrier substrate may be attached to the device substrateusing fusion, diffusion, eutectic, anodic, polymer, and/or othersuitable bonding methods. Example carrier substrates include silicon,glass, and quartz. The carrier substrate 402 may include otherfunctionality such as, interconnect features, wafer bonding sites,defined cavities, and/or other suitable features. The carrier substratemay be removed during subsequent processing (e.g., after thinning).

In operation 254 of FIG. 2B, the semiconductor substrate is thinned. Thedevice substrate is flipped and thinned using wet etch processes, dryetch processes, plasma etch processes, chemical mechanical polish (CMP)processes, and/or other suitable processes for removing portions of thesemiconductor substrate. Example etchants suitable for thinning thesubstrate include HNA (hydrofluoric, nitric, and acetic acid),tetramethylammonium hydroxide (TMAH), KOH, buffered oxide etch (BOE),and/or other suitable etchants compatible with CMOS process technology.

In FIG. 5, the device substrate is thinned such that the bulk siliconlayer is removed. In other embodiments both the bulk silicon layer andthe buried insulating layer are removed. The device substrate may bethinned in a plurality of process steps, for example, first removing thebulk silicon layer of an SOI wafer followed by removal of a buriedinsulating layer of the SOI wafer. In an embodiment, a first thinningprocess includes removal of the bulk silicon using, for example,grinding, CMP, HNA, and/or TMAH etching, which stops at the buried oxidelayer. The first thinning process may be followed by a second thinningprocess, such as BOE wet etch, which removes the buried oxide and stopsat the silicon of the active layer. The thinning process may expose anactive region of the substrate. In an embodiment, a channel region(e.g., active region interposing the source/drain regions and underlyingthe gate structure) is exposed. The substrate may have a thickness ofapproximately 500 Angstroms (A) to 1500 A after the thinning process.For example, in one embodiment the active layer of an SOI substrate hasa thickness of between of approximately 500 A and 1500 A.

In other embodiments, the device substrate is thinned such that the bulksilicon layer is removed, and at least a portion of the buriedinsulating layer remains on the substrate as shown in FIG. 5. Theremoval of the bulk silicon may be performed using, for example, CMP,HNA, and/or TMAH etching, which stops at the buried insulating layer.The substrate may have a thickness of approximately 500 Angstroms (A) to15000 A after the thinning process. For example, in one embodiment theactive region of an SOI substrate has a thickness of between ofapproximately 500 A and 1500 A. The buried insulating layer (nowproviding the surface of the substrate) may be the isolation layer andhas a thickness between about 1000 A to a few microns.

In operation 256 of FIG. 2B, a trench is formed on the substrate toexpose and provide contact to one or more of the conductive traces ofthe MLI structure. The trench may be formed by photolithographyprocesses to pattern the trench opening followed by suitable wet, dry orplasma etching processes. In an embodiment, the trench exposes a portionof a metal one (metal 1) layer of the MLI (e.g., the first metal layerformed in the MLI structure after the gate structure is formed).Referring to the example of FIG. 6, a trench 602 is etched, specificallythrough the active layer 308, to expose a landing region on a conductiveline 320 of the MLI structure 318. Alternatively, the trench may beetched through the isolation region 306 (e.g., oxide).

In operation 258 of FIG. 2B, an isolation layer is formed on thesubstrate. The isolation layer may include a dielectric material such asan oxide or nitride. In an embodiment, the isolation layer is siliconoxide. Referring the example of FIG. 7A, an isolation layer 702 isdisposed in the trench 602 and over the insulating layer 306. In anembodiment, the isolation layer 702 is silicon dioxide. As discussedabove, in some embodiments, an isolation layer is not formed over theinsulating layer if the insulating layer of the SOI substrate wasremoved during the substrate thinning process. FIG. 7B includes anisolation layer 702 formed in the trench 602 and over the active layer308 of the of the SOI substrate. The following FIGS. 8-14 illustrate anembodiment wherein BOX layer 306 was removed in the substrate thinningprocess, such as shown in FIG. 7B. The teaching relating to thesefigures is equally applicable, however, to embodiments in which all or aportion of BOX 306 (referred to hereafter as insulating layer 306)remains, as shown in FIG. 7A.

In operation 260 of FIG. 2B, an interconnect layer is formed andpatterned on the isolation layer 702. One or more openings are patternedand etched in the isolation layer 702 to expose underlying metal orconductive areas. The interconnect layer may provide a connection (e.g.,I/O connection) to the MLI structure. The interconnect layer may providea connection (e.g., I/O connection) to the transistor. The interconnectlayer may include a conductive material such as, copper, aluminum,combinations thereof, and/or other suitable conductive material. Theinterconnect layer may provide functions as a re-distribution layer(RDL). The interconnect layer is formed using metal deposition orplating techniques and then patterned. Referring to the example of FIG.8, an interconnect layer 802 is disposed on the insulating layer 702.The interconnect layer 802 may provide a signal input/output to theBioFET as well as connecting to the MLI through the trench 602. In anembodiment, the interconnect layer 802 includes an aluminum copperalloy.

In operation 262 of FIG. 2B, a passivation layer is formed on the devicesubstrate. The passivation layer may cover portions of the interconnectlayer. The passivation layer may include openings where a bond (e.g.,I/O) may be formed. In an embodiment the passivation layer includessilicon dioxide, however, other compositions are possible. Thepassivation layer may be suitable to provide protection of the device,e.g., the interconnect layer, including from moisture. Referring to theexample of FIG. 9, a passivation layer 902 is formed on the substrateincluding on the interconnect layer 802. The passivation layer 902includes an opening 904 where a bond (e.g., wire bond, bump) may provideconnection (e.g., I/O connection) to the device 300. In other words, theopening 904 may expose a conductive I/O pad.

In operation 264 of FIG. 2B, an opening is formed on the backside of thesubstrate. The opening is formed such that a portion of the activeregion of the substrate underlying the transistor structure (e.g.,channel region) is exposed. The opening is substantially aligned withthe active region of the transistor and may be aligned with the backgate structure 312/314. The opening may be formed by suitablephotolithography processes followed by an etching process such as a dryetch, wet etch, plasma etch, and/or combinations thereof. In someembodiments, the opening is formed in the isolation layer. In otherembodiments, the opening is formed in the buried insulator layer (of theSOI substrate). Referring to FIG. 9, an opening 906 is provided in theisolation layer 702. The opening 906 exposes a portion of the activelayer 308. In particular, an active region 319 and portions ofsource/drain regions 316 may be exposed.

Referring back to FIG. 2A, in operation 207, an exposed substrate areain the opening is treated. The treatment includes at least one of animplant process, a diffusion process, and an anneal process. Animplantation process embeds dopants into the surface of the substrate.The depth of the implantation is controlled by an energy of theimplantation process. The concentration of dopant in the substratedepends on a dosage of the implantation. Referring to FIG. 10, theimplantation process creates a treated layer 1002 at the bottom surfaceof the opening 906 that has an overall lower net dopant concentrationthan the rest of the active region 319 below the bottom surface of theopening 906. To achieve an overall lower net dopant, a dopant of anopposite conductivity type from the active region 319 is implanted. Fora n-type MOS, arsenic or phosphorous is implanted. For a p-type MOS,boron is implanted. Because these dopants have an opposite conductivitytype from the active region 319, the overall net dopant concentration isreduced at the surface of the active region 319. The treated layer isthen a lightly doped channel layer as compared to the rest of the activeregion. If sufficient dopants are implanted, the treated layer is then adepleted layer. A relatively low energy implantation process may be usedto confine the dopants to a surface layer. For example, the implantationenergy may be less than about 10 keV or less than about 15 keV. If theactive region 319 is sufficiently thick and a larger treated layer is tobe created, a higher energy may be used. According to variousembodiments, the treated layer has a peak concentration at about 5angstroms or a few hundred angstroms from the surface. A thickness ofthe treated layer may be between about 10 nanometers to a few hundrednanometers.

The implantation process may be performed directly on the substrate orthrough a mask. An implantation mask may be formed first by depositing asacrificial oxide layer, which is then patterned to form an opening forthe implantation. The mask creation may be performed with operation 206where opening 906 is formed. In some embodiments, the opening 906 islarger than the implantation opening. For example, the treated layer1002 may extend to a portion of the source drain 316 or be confined to asurface of the active region 319.

In some embodiments, the insulation layers 306 and passivation layers902 are sufficient to block the dopants from embedding in other portionsof the BioFET. In one embodiment, the operation 262 of FIG. 2B isperformed without forming the opening 904 to protect the interconnectlayer 802 from the implantation. In these embodiments, openings 904, 906in the passivation layer 902 are formed after the implantation.

After the implantation, the substrate is annealed to activate thedopant. Different dopants require different amounts of annealing toactivate. Lower temperature anneals activate at a reduced rate. Becausethe activation anneal occurs after the MLI 318 and interconnect layer802 are formed, the stability and contamination of the metal material inthe device are balanced against the activation rate. In someembodiments, the implantation and activation anneal are performed beforethe interconnect layer 802 is formed. The activation anneal may beperformed at about 400 degrees Celsius, about 450 degrees Celsius, andmay be less than about 500 degrees Celsius. In some embodiments, a laseris used to activating the dopants. Because the laser energy may befocused at the surface of the substrate and the laser exposure is veryshort in duration, often less than one microsecond, the laser activationmay be performed without significant adverse effects to the much deeperMLI 318. In one embodiment, a laser beam scans the die. In anotherembodiment, a laser beam is adjusted to have a size that is sufficientto activate the dopants one die at a time.

Alternatively, the treated layer 1002 may be formed by adding a dopantthat tends to deactivate the primary dopant of the active region 319. Ina nMOS example, hydrogen may be added to create a treated layer 1002because hydrogen can deactivate boron. The hydrogen may be implantedjust as arsenic, phosphorous, and boron. Hydrogen may also be added by adiffusion process. One diffusion process involves annealing in ahydrogen environment (hydrogen/deuterium gas or forming gas) or applyinghydrogen plasma to the surface. Another diffusion process involvesdepositing a highly-doped dielectric layer in the opening 906 over theactive region 319 and then annealing for the doped hydrogen to diffuseinto silicon. The highly-doped dielectric layer may be a silicon oxideor a silicon nitride film. After the diffusion anneal, the dielectriclayer is removed.

In addition to the implantation and diffusion methods to form thetreated layer 1002, a treated layer 1002 may be formed by annealing inan oxygen or ozone environment. The anneal repairs dangling bonds causedby plasma processes Anneals in an oxygen atmosphere of oxygen or ozonerepairs dangling bonds. A treated layer 1002 may also be formed byannealing in a hydrogen environment. Anneals in a hydrogen atmosphere ofhydrogen or deuterium mitigates mobile ions from plasma-induced damage.Anneals for mitigation of mobile ions are at a lower temperature thanthe diffusion anneal described above and may be combined into one step.

The treated layer 1002 allows electrical property tuning of the BSSBioFET. When the treated layer 1002 is a lightly doped layer or adepleted layer, the BSS BioFET can be made more sensitive to moleculesbound to receptors, improving the transconductance of the BSS BioFET. Inother words, the drain current for a gate voltage may be increasedrelative to a BioFET without treated layer 1002. In some embodiments,the treated layer 1002 provides a larger bandgap which can avoid orreduce current leakage. In some embodiments, the treated layer 1002includes fewer defects than untreated layers and can reduce device noisefrom mobile ions and interfacial charges. A number of BioFETs on a samedevice may be tuned to different sensitivities with for the same ordifferent bio-entities by changing the process of forming the treatedlayer. For example some of the BioFETs may have a treated layer having afirst dopant at a first concentration and other BioFETs may have atreated layer having a second dopant at a second concentration. Thedifferent treated layer allows the BioFETs to detect targetsdifferently. By using different masks and separate lithography steps,more than one type of treated layers can be formed on one device.

Referring back to FIG. 2A, in operation 208 a dielectric layer is formedin the opening. The dielectric layer is formed on the exposed substrateunderlying the gate structure of the FET and covers the entire bottom ofthe opening 906 over the treated layer 1002. Exemplary dielectricmaterials include high-k dielectric films, metal oxides, and/or othersuitable materials. Specific example of dielectric materials includeHfO2, Ta2O5, Au2O3, WO3, oxides of Pt, Ti, Al, and Cu, and otherdielectrics such as SiO2, Si3N4, Al2O3, TiO2, TiN, SnO, SnO2, amongothers. The dielectric layer may be formed using CMOS processes such as,for example, chemical vapor deposition (CVD), plasma-enhanced chemicalvapor deposition (PECVD), atmospheric pressure chemical vapor deposition(APCVD), low-pressure CVD (LPCVD), high density plasma CVD (HDPCVD), oratomic layer CVD (ALCVD). In some embodiments, the dielectric layerincludes a plurality of layers. For example, a dielectric layer mayinclude a hafnium oxide layer over an aluminum oxide or titanium oxidelayer. In the example of FIG. 11, a dielectric layer 1102 is disposedover the active layer 319 and a portion of the source and drain 316. Thedielectric layer 1102 can be patterned to be aligned with the gatestructure (e.g., is disposed and patterned to remain only in the opening906.)

Referring back to FIG. 2A, in optional operation 210 a metal layer isdeposited. The metal layer may be an elemental metal, metal alloy, or aconductive metallic compound. Suitable elemental metals includetantalum, niobium, tungsten, ruthenium, aluminum, zirconium, vanadium,titanium, cobalt, molybdenum, osmium, chromium, rhodium, gold,palladium, rhenium, nickel, or other transition metal commonly used insemiconductor process. The metallic compound includes conductivenitride, silicide, and oxides of these transition metals. For example,tungsten nitride, tantalum nitride, and ruthenium oxide. The metal layermay be a composite layer of two or more layers. For example, the metallayer may include both tungsten nitride and ruthenium oxide.

The metal layer is deposited conformally over the substrate and in theopening covering the interface layer. The metal layer may be depositedusing a PVD (sputtering), metal chemical vapor deposition (MCVD), atomiclayer CVD (ALCVD), electrochemical deposition with a seed layer, orelectroless deposition. In some embodiments, an ion beam deposition maybe used to selectively deposit the metal layer in and around theopening.

In optional operation 212, the metal layer is patterned to form a metalcrown structure. In some embodiments, the patterning involves removing,by etching, unwanted portions of the metal layer deposited in operation210. An etch mask is first deposited and patterned. The etch mask may bea photoresist or a hardmask patterned by a photolithography process. Inother embodiments, a photoresist material is first deposited andpatterned on the substrate and removed after the metal layer isdeposited. Lifting off the photoresist material also removes anyoverlying metal layers. The lift-off technique may be useful when thedry etch involving plasma to remove the metal pattern would causeundesirable amounts of plasma-induced damage to other exposed metalsurfaces. Because the photoresist in the lift-off process may be removedwith just wet etching or including low-power plasma etching, it issometimes preferred over the metal patterning technique. However, thelift-off process has the potential to produce more contaminants and ashape of the resulting metal crown structure may include jagged edges.

In the example of FIG. 12, a metal crown structure 1202 is disposed overa dielectric layer 1102 in and around the opening. As shown, the metalcrown structure 1202 includes a lip that overlaps a portion of theisolation layer 702. In some embodiments all of the metal crownstructure 1202 is within the opening 906 of FIG. 11. In otherembodiments, the dielectric layer 1102 and metal crown structure 1202consumes the volume of the opening as shown in FIG. 12.

Referring back to FIG. 2A, in operation 214 a microfluidic channel orwell is disposed on the device substrate. The fluidic channel defines aregion overlying the metal crown structure through which the analyteflows. The fluidic channel may be formed by lithography utilizing SU-8(an epoxy negative photoresist), wafer bonding methods, and/or othersuitable methods. Referring to the example of FIG. 13, a fluidic channel1302 is disposed on the substrate. The fluidic channel 1302 provides awell 1304 overlying the metal crown structure 1202.

Referring back to FIG. 2A, in operation 216, a receptor or filmtreatment is disposed on the metal crown structure. The receptor mayinclude enzymes, antibodies, ligands, protein, peptides, nucleotides,and portions of these. The receptor may be a modified form of a nativeprotein or enzyme configured on one end to detect a specific analyte.The other end of the receptor is configured to bond to the metal crownstructure or another molecule/film treatment that is bonded to the metalcrown structure. As shown in FIG. 14, a plurality of receptors 1402 isdisposed on metal crown structure 1202. By using a metal crownstructure, a larger surface area is available to receptors for bondingand hence more sites are available for biomolecule or bio-entitydetection. If the metal crown structure is not used, then the receptorsare disposed on the dielectric layer 1102 directly or through anothermolecule/film treatment. Operation 216 may be performed before operation214 in certain embodiments.

The embodiments of FIG. 2B pertain to aspects of the present disclosurewhere the electrical connections for the BioFET device are made on thesame side of the substrate as the fluidic connections. The presentdisclosure also pertains to embodiments where the electrical connectionsfor the BioFET device are made to the opposite side of the substrate asthe fluidic connections. In those embodiments, electrodes and pads areformed connecting to the MLI on the front side of the substrate beforethe carrier substrate is bonded and device substrate thinned From thebackside, trench 602 is not formed.

During operation of the BioFET device, a solution that contains targetmolecules is provided in the fluidic channel. The BioFET device maycontain different areas for processing the target molecule. Somebio-material may be lysed, separated, dyed, and otherwise tested oranalyzed using chemical, electrical, or optical means. For example, adrop of blood may be inserted in an inlet and initially separated byplasma and cell type. Certain cells in the blood drop may be lysed. Somemacromolecules in the lysate may be further broken down for analysisdownstream in the flow path. Deoxyribonucleic acid (DNA) molecules maybe fragmented by enzyme reaction, restriction or shearing into targetstrands.

After processing the bio-material into targets, the targets are detectedby flowing through microfluidic channels and wells containing theBioFETs. Either the dielectric layer 1102 or the metal crown structure1202, if used, is the sensing surface of the BioFET. The flow may becontrolled such that the targets have a long residence time in thepresence of the sensing surfaces as compared to the reaction time. Insome embodiments, one or more gate bias is varied while the currentflown through the BioFET is collected. The electrical information fromthe BioFET are collected and analyzed.

In various embodiments, a CMOS fabrication facility (e.g., foundry) mayprocess the methods in accordance with various embodiments for theassociated device up to the fluidic channel formation. In an embodiment,a subsequent user may provide the surface treatment technologies, ionicsolutions, receptors, and the like.

In summary, the methods and devices disclosed herein provide a BioFETthat is fabricated using CMOS and/or CMOS compatible processes. Someembodiments of the disclosed BioFET may be used in biological and/ormedical applications, including those involving liquids, biologicalentities, and/or reagents. One detection mechanism of some embodimentsdescribed herein includes a conductance modulation of the FET of theBioFET due to the binding of the target bio-molecule or bio-entity tothe fluidic gate structure, or a receptor molecule disposed (e.g.,immobilized) on the fluidic gate structure of a device.

Some embodiments of the BioFETs are arranged in an array form. The gatestructures may be built on silicon-on-insulator (SOI) substrates. Thismay provide advantages in some embodiments of operation at a higherspeed and/or consumption of less power. The inverted transistor providedon an SOI substrate may achieve improved fabrication uniformity, haveimproved process control, and increase the BioFET density. Someembodiments may provide for an improved short-channel effect, forexample, due to the formation on a SOI substrate. Other features includelower current leakage, lower power consumption, and lower device noisefrom irradiation processes.

Thus, it will be appreciated that in one embodiment a BioFET device isdescribed that includes a substrate, a transistor structure in thesubstrate including a treated layer next to a channel region in theactive region, an isolation layer with an opening on a side of thesubstrate opposite from a gate structure of the transistor, and andielectric layer in the opening. The transistor structure has a gatestructure over a source region, a drain region, and an active regionincluding a channel region and a treated layer.

One aspect of the present disclosure pertains to a semiconductor deviceis provided that includes an array of BioFET devices. A first and secondplurality of BioFET devices in the array include an active regionbetween a source and a drain region and underlying a gate structure. Theactive region including a channel region that adjoins the gate structureand a treated layer. The treated layer in the first plurality of BioFETdevices has a first dopant at a first concentration. The treated layerin the second plurality of BioFET devices has a second dopant at asecond concentration. The first and second plurality of BioFET devicesalso include a dielectric layer disposed on a side of the treated layeropposite from the channel region.

Another aspect of the present disclosure pertains to a method offabricating a BioFET device includes forming a transistor on asemiconductor substrate, etching an opening in an isolation layerdisposed on a second side of the semiconductor substrate that exposesthe active region of the transistor, embedding a dopant into the activeregion of the transistor through the bottom of the opening to form atreated layer, and depositing an dielectric layer on the treated layer.The embedding may be accomplished by implant a dopant having an oppositeconductivity from a dopant in the channel region, implanting hydrogen,and diffusing one or more dopants through annealing a highly dopedsacrificial layer. The method may also include annealing thesemiconductor substrate in an oxygen or hydrogen environment.

In describing one or more of these embodiments, the present disclosuremay offer several advantages over prior art devices. In the discussionof the advantages or benefits that follows it should be noted that thesebenefits and/or results may be present is some embodiments, but are notrequired. Advantages of some embodiments of the present disclosureinclude the ability to offer a customer-customizable product. Forexample, fluidic channel formation, receptor introduction and the likemay be performed by a customer. As a further example of advantages ofone or more embodiments described herein, in conventional devices it istypical to require high aspect ratio processing to form a bio-compatibleinterface (e.g., requiring etching from a front surface of the substrateto a gate structure). Because the present methods provide for processingon a backside of a thinned wafer, the aspect ratio is reduced.

What is claimed is:
 1. A biological field-effect transistors (BioFET)device, comprising: a substrate; a transistor structure in the substratehaving a gate structure over a source region, a drain region, and anactive region, the active region including a channel region and atreated layer; an isolation layer on a side of the substrate oppositefrom the gate structure, said isolation layer having an opening at theactive region of the transistor structure; and a dielectric layer in theopening.
 2. The BioFET device of claim 1, wherein the treated layer is alightly doped layer.
 3. The BioFET device of claim 1, wherein thetreated layer comprise a dopant having a conductivity type opposite froma dopant in the channel region.
 4. The BioFET device of claim 1, whereinthe treated layer comprises hydrogen.
 5. The BioFET device of claim 1,further comprising a metal crown structure over the isolation layer andat least partially covering sidewalls of the opening.
 6. The BioFETdevice of claim 1, wherein the dielectric layer comprises aluminumoxide, titanium oxide, hafnium oxide, tantalum oxide, tin oxide, or acombination of these.
 7. The BioFET device of claim 1, furthercomprising: a fluidic channel disposed on the isolation layer.
 8. TheBioFET device of claim 1, further comprising: a multi-layer interconnect(MLI) disposed in the substrate on a side of the substrate that is sameas the gate structure.
 9. The BioFET device of claim 8, wherein acarrier substrate is bonded to the substrate via a passivation layerover the MLI.
 10. A method of making a BioFET device, comprising:forming a transistor on a semiconductor substrate, wherein thetransistor includes a gate structure formed on a first side of thesemiconductor substrate and an active region between a source region anda drain region; etching an opening in an isolation layer disposed on asecond side of the semiconductor substrate, wherein the opening exposesthe active region of the transistor; embedding a dopant into the activeregion of the transistor through the bottom of the opening to form atreated layer; and, depositing a dielectric layer on the treated layer.11. The method of claim 10, wherein the embedding a dopant comprises:implanting a dopant having an opposite conductivity from a dopant in theactive region.
 12. The method of claim 11, wherein the embedding adopant further comprises: forming an implant mask; activating thedopant; and removing the implant mask.
 13. The method of claim 10,wherein the embedding a dopant comprises: implanting hydrogen ordeuterium when the transistor is an n-type transistor.
 14. The method ofclaim 10, wherein the embedding a dopant comprises: forming ahighly-doped sacrificial dielectric layer in the opening; diffusing adopant from the sacrificial dielectric layer to the active region; andremoving the sacrificial dielectric layer.
 15. The method of claim 11,further comprising: annealing the semiconductor substrate in an oxygenor hydrogen/deuterium environment.
 16. The method of claim 10, furthercomprising: thinning the semiconductor substrate; and depositing anisolation layer on a second side of the semiconductor substrate.
 17. Themethod of claim 16, wherein the semiconductor substrate is an SOIsubstrate and the thinning at least partially removes a buried oxidelayer.
 18. The method of claim 10, further comprising: forming a metalcrown structure over the dielectric layer, wherein a portion of themetal crown structure covers a portion of the isolation layer; andbinding a receptor on the metal crown structure, wherein the receptor isselected from the group consisting of enzymes, antibodies, ligands,receptors, peptides, nucleotides, cells of organs, organisms and piecesof tissue.
 19. A device, comprising: a plurality of first BioFETs, eachfirst BioFET including: an active region between a source and a drainregion and underlying a gate structure, the active region including achannel region and a first treated layer and wherein the channel regionadjoins the gate structure; and a dielectric layer disposed on a side ofthe first treated layer opposite from the channel region; wherein thefirst treated layer includes a first dopant at a first concentration;and a plurality of second BioFETs, each second BioFET including: anactive region between a source and a drain region and underlying a gatestructure, the active region including a channel region and a secondtreated layer and wherein the channel region adjoins the gate structure;and a dielectric layer disposed on a side of the second treated layeropposite from the channel region; wherein the second treated layerincludes a second dopant at a second concentration.
 20. The device ofclaim 19, wherein the first BioFET is an n-type transistor and the firstdopant is hydrogen and wherein the second BioFET is a p-type transistorand the second dopant is boron.